Examine source code of rfcrecenter

{
  "author": "arxaqapi",
  "manifest_version": 2,
  "name": "rfcrecenter",
  "version": "1.0",
  "description": "Recenters the rfc content div for better reading",
  "icons": {
    "48": "icons/i-48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://tools.ietf.org/html/rfc*",
        "*://tools.ietf.org/html/*"
      ],
      "js": [
        "rfcrecenter.js"
      ]
    }
  ]
}